In the handling and treatment of certain moving indefinite length materials, it is desirable and often essential that tension on the moving material between points in its longitudinal path of travel be controlled and/or maintained at a substantially constant level. When materials of an elastic nature, such as textile fabrics and yarns, are longitudinally conveyed under tension between fixed feeding points for the material, the materials may irregularly stretch or contract in length due to treatments applied thereto or as a result of structural irregularities which may be present along the length of the material itself. Such factors can create variations in tension in the material between the fixed feeding points and can result in non-uniform treatment of the materials. For example, when a textile pile fabric, such as a carpet is subjected to a bushing operation to raise the pile components of the fabric prior to a shearing operation, it is desirable that the moving fabric be under substantially constant pressure contact with the bushing means to ensure uniform raising of the pile surface. Similarly, in heat treatment of certain moving materials, such as textile fabrics and yarns, substantially constant tension is necessary to control shrinkage or stretching of the materials during the treating operation.
It is a conventional practice in such material handling operations to provide control devices for maintaining a desired tension on the moving material in portions of its path of travel. Typically, such tension control devices comprise displaceable material guiding rollers, commonly referred to as floating rollers or dancer rollers, which are positioned to engage the material in its path of travel between fixed material-feeding points. The movable guide rollers are displaced in response to an increase or decrease in the length of the material caused by tension variations, and their movement signals a change in speed of one of the feeding rollers to adjust tension to the desired level. Certain other displaceable roller tension control devices are operatively connected to pneumatic piston means which exerts a predetermined force or pressure on the roller to move the same and increase or decrease the length of the material path, thereby maintaining a substantially constant tension on the material. U.S. Pat. Nos. 3,083,887 and 3,326,436 illustrate tension control devices of the floating or dancer roller type.
Other tension control devices employ axially displaceable, driven feed rollers which are displaced by tension variations in the material, with such displacement operating to proportionally vary the speed of the feed rollers through variable speed transmission arrangements to return the tension to a desired level. U.S. Pat. No. 2,787,463 discloses a web tension control mechanism comprising a pair of positively driven web feeding rollers mounted for movement in a linear direction in response to variations in tension in the web. The rollers are driven by a variable-pitch pulley and belt drive arrangement, and an adjustable spring mechanism is employed to establish a desired pressure against displacement of the rollers in one direction. Any variation in tension in the web displaces the driven feed rollers to cause corresponding displacement of the pulley belt on the variable pitch pulley to increase or decrease the speed of the rollers and maintain a substantially uniform tension on the material.
U.S. Pat. No. 2,759,728 discloses a tension control system wherein a pair of feed rollers for moving a web of material are mounted for displacement on a lever arm having a spring biasing arrangement. Variation in tension in the web against the force of the spring causes corresponding displacement of the driven rollers to vary the position of a roller drive belt on a variable-pitch drive pulley which correspondingly varies the speed of the rollers to maintain the tension on the web at the desired level.
Other tension control devices employing variable-speed drive motors, variable-pitch drive pulley arrangements, or floating roller tension sensing devices are disclosed in the following U.S. Pat. Nos. 1,275,636; 1,692,955; 1,886,342; 2,029,854; 2,301,249; 2,637,991; 2,847,210; 2,897,754; 2,914,266; 3,167,265; and 3,771,744.
Generally such tension control devices as described in the aforementioned patents which require axial displacement of material feed or dancer roller arrangements to sense and compensate for tension variations in the material path not only add to the cost of the material handling equipment, but also require additional space to allow for lengthening and shortening of the material handling path. Such arrangements, because they require changes in the length of the path of the material between fixed points along the path to sense and/or correct variations in tension, are often undesirable, particularly where the material between fixed material feeding points must be in a precise position of contact with a material treating device, such as a brush or a knife coating blade, or where portions of the moving material must be exposed to multiple treating operations at precisely timed intervals.